1. Field of the invention.
The present invention relates to a copying machine with a device for imparting motion to the carriage carrying the original, in particular, to a device for establishing the correct velocity of the carriage.
2. Description of the prior art
It is known, for example from U.S. Pat. No. 3,900,256, for the original which is to be reproduced to be located on a carriage having a transparent plane surface, which is transported over a scanning slit arranged transversely with respect to the motion of the carriage, so that a transverse strip of the original is illuminated. The corresponding image is transmitted through a fixed optical system which is located below and adjacent to this slit on to a photo-sensitive element which is moved at a velocity substantially equal to, or is kept at a constant ratio with respect to the velocity of the carriage.
The photosensitive element may be a cylinder having a photoconducting layer on its outer surface which is electrostatically charged and on which the latent electrostatic image is formed, to be developed and transferred onto a copy of page of plan paper, or the copy sheet itself may have one face coated with a photoconducting material which is electrostatically charged and, after exposure to the image of the original, carries a latent image which is developed using developing powder or liquid and fixed on the copy sheet, as is known in the Electrofax copying process.
Whatever the type of process used and consequently the type of photo-sensitive element used, copying machines which employ a movable carriage do suffer from the disadvantage that whilst the motion of the photosensitive element is taking place, at a constant velocity, at least during exposure to the luminous image, the velocity of motion of the carriage is not constant to the same degree, both as a result of play in the parts providing for transmission and conversion of the rotational movement of a drive shaft to a translatory motion of the carriage, and a result of play, wear, and irregularities in the guides on which the carriage slides.
Strict constancy of the velocity ratio between the carriage and the photo-sensitive element is absolutely essential in order to ensure that the image of the original is transferred to the photo-sensitive element without any geometrical distortion.
Electronic control systems for providing constancy of the carriage velocity suffer from the disadvantage of being too expensive, all the more so because the use of a movable carriage for transporting the original is a characteristic of less expensive copying machines, namely those machines which fall in the medium to low price range of the market for reprographic machinery.